In positron emission tomography (PET) apparatus, scintillation detectors characterized by high sensitivity and rapid response have been adopted, by virtue of their ability of coincidence counting of gamma ray (annihilated gamma ray: 511 KeV) of relatively large energies. Detector characteristics required herein include high time resolution for ensuring high counting rate and removal of random noise, and also include excellent energy resolution for ensuring removal of scattered ray from inside the body.
Therefore, demands for a scintillator suitable for the detectors satisfying these requirements include high density and high atomic number (largeness in photoelectric absorption ratio) in view of detection efficiency, high level of emission energy in view of needs for rapid response and high energy resolution, and short fluorescence lifetime (fluorescence decay time). The recent systems have adopted dense arrangement of a large number of fine, thin and long scintillators, aiming at multi-layered configuration and higher resolution, so that also their handlability, workability and price have become important matters to be considered for their selection.
Tl:NaI had most generally been used for the scintillation detector by virtue of its large energy of emission and relatively low cost, but was not expectable for improvement in sensitivity of the detector due to its low density, inconvenient to handle due to its deliquescence, and has therefore been replaced with Bi4Ge3O12 (BGO).
BGO is characterized by a wavelength of 490 nm, a refractive index of 2.15, and a density of 7.13 g/cm3 twice as large as that of Tl:NaI, and consequently shows a large ray energy absorption coefficient. Unlike hygroscopicity of Tl:NaI, BGO has no hygroscopicity, and has an advantage of good workability. Disadvantages reside in that BGO has a fluorescence conversion efficiency of only as small as κ% of that of Tl:NaI, so that light output in response to gamma ray is smaller than that of Tl:NaI, and also in that BGO shows an energy resolution in response to 1-MeV gamma ray of 15%, in contrast to 7% shown by Tl:NaI. Still another disadvantage is that the fluorescence decay time is extremely as long as 300 nsec or around.
Ce:Gd2SiO5(Ce:GSO), developed in Japan, is slightly inferior to BGO in the detection sensitivity, but is understood as a well-balanced, high-performance scintillator characterized by its density (6.71 g/cm3), energy of light emission (doubled value of BGO), response time (30 to 60 nsec) and radioactive ray resistance (>105 gray). However, problems reside in slightly slower rise-up time, positive-hysteresis to radioactive ray (property of increasing the energy of light by irradiation), and strong tendency of cleavage.
A scintillator crystal, supposedly as being the state of the art at present, is Ce-doped Lu2SiO5 (Ce:LSO), characterized by excellent scintillator characteristics including high density (approx. 7.39 g/cm3), short lifetime (approx. 50 nsec), and large emission energy (three times or more of BGO). The LSO crystal can be manufactured by the Czochralski method, and therefore has a some-ten-billon dollar market mainly contributed by US companies such as CTI Molecular Imaging Inc. (CTI), Crystal Photonics Inc. (CPI) and so forth. On the other hand, problems reside in high costs for manufacturing and processing due to its melting point relatively as high as 2,150° C. and large anisotropy in the linear expansion coefficient, and in low yield. Growth of single crystal of high-melting-point oxide from its molten state is generally carried out using a metal called iridium (Ir) as a crucible material, wherein temperatures exceeding 2,000° C. are close to the softening point of Ir, so that a severe temperature control is necessary for manufacture of the LSO crystal. In addition, the lifetime of the Ir crucible is short, and huge costs for re-casting the crucible fall heavily on the manufacturers. Moreover, large output is necessary for the high-frequency oscillator in order to realize such high super-high temperature, and this also pushes up the running cost as a whole.
On the other hand, Ce:GSO and Ce:LSO, having been used as light emitting materials for scintillator, are large in the emission energy by virtue of a large amount of Ce, a light emitting element, contained therein, but the content exceeding several percents may result in distinct concentration quenching, and no more shows the scintillator effect.
In addition, Ce is, second to La, largest in the rare earth ions, and is significantly larger than representative rare earth ions (Y, Gd, Lu) in the host crystal, so that the effective segregation coefficient largely deflects from 1. This means that compositional fluctuation in the direction of growth is inevitable. This phenomenon is causative of varying physical properties such as emission energy and so forth, and raises a serious problem when adoption to a high-precision-type PET is aimed at.
In this situation, a current expectation is directed to development of a next-generation scintillator advantageous not only in the cost, but also in having a larger energy absorption coefficient, and higher energy resolution and time resolution, that is, capable of increasing the number of sampling in a unit time (Patent Document 1).
On the other hand, not only PET, but also X-ray CT holds a large importance in medical imaging apparatus. When taking the whole range of non-destructive inspection into consideration, also X-ray CT, and scintillator crystal for radioactive ray transmission inspection are of large importance. Scintillator crystals aimed at these purposes are desired to have large emission energy such as Tl:NaI and CsI, rather than to have short fluorescence lifetime such as Ce:GSO and Ce:LSO.
From these points of view, current demands are directed to develop a next-generation scintillator characterized by low cost, high energy absorption coefficient, and large emission energy.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 2001-72968